As disclosed in Japanese Laid-Open Patent Publication No. 2007-049025 (Patent Document 1), an inkjet recording apparatus including an inkjet head and capable of outputting a two-dimensional image is known. The inkjet head includes a plurality of channels for ejecting inkjet ink (which hereinafter may be simply referred to as “ink”). The plurality of channels eject ink while moving relative to a recording medium such as paper and cloth. The known systems for ejecting ink include a pressure system with a variety of actuators, such as piezoelectric actuator, electrostatic actuator, and actuator using thermal deformation, and a thermal system generating bubbles by heat.
Among those systems, the system including a piezoelectric actuator is advantageous in that output is large, modulation is possible, the responsiveness is high, and any ink can be used. As piezoelectric actuators, those formed of piezoelectric substances, such as lead zirconate titanate-based compounds (PZT), are known. In recent years, MEMS (Micro Electro Mechanical System) devices with silicon (Si) substrates have been increasingly employed. More specifically, MEMS techniques are applied to fabrication of piezoelectric actuators so as to form a piezoelectric substance into a thin film, and thereby, a piezoelectric actuator can be formed of a thin-film piezoelectric element.
When a piezoelectric actuator is formed of a thin-film piezoelectric element, high-precision processes using semiconductor process techniques, such as film deposition and photolithography, can be performed to achieve size reduction and higher density. A batch process can be performed on a wafer with a large area. Therefore, high production efficiency and cost reduction can also be expected. Another advantage is that conversion efficiency between mechanical energy and electrical energy is improved to provide high characteristics. It therefore can be said that thin-film piezoelectric elements are suitable for implementing high-resolution, compact, and low-cost inkjet heads.
Here, when a piezoelectric actuator is formed of a thin-film piezoelectric element, typically, an electrode is formed on a silicon (Si) or other substrate, and a piezoelectric layer of a thin film is formed on the electrode. Here, since the piezoelectric layer has a lattice constant different from that of a crystal in the electrode, a polycrystal made of a group of a plurality of crystals is formed. Depending on the production process, the polycrystal (polycrystal film) may have a granular crystal shape or a columnar crystal shape.
In the case of granular crystal, the polycrystal film forms granular crystals having a grain size of a few hundred nm. On the other hand, in the case of columnar crystal, the polycrystal film has a width of a few hundred nm and forms crystal grains that gather into a shape like a single elongated column in the film thickness direction. In the case of columnar crystal, it is known that the more crystals are grown on the crystal plane positioned at the same location in the film thickness direction (in other words, the higher is the orientation), the higher is the piezoelectric characteristics of the polycrystal film.